Transcript Accelerometer Concepts

Accelerometry
Newton’s 2nd Law
 The Law of Acceleration
 Describes how a body moves when an external force is
applied
 “An external force will cause the body to accelerate in
direct proportion to the magnitude of the force and in
the same direction as the force”
 F=ma
 If a body’s mass is assumed to be constant, then a
measured acceleration is an indicator of a change in
the force applied to the body.
Direct measurement by
accelerometry
 Accelerometers are devices that measure applied
acceleration acting along a sensitive axis which can be
used to measure the rate and intensity of body
movement in up to three planes (anterior–posterior,
mediolateral and vertical)
 Accelerometers can also be used to measure tilt (body
posture) making them superior to those devices that
have no ability to measure static characteristics.
Direct measurement by
accelerometry
 Although each type of accelerometer uses different mechanisms to
measure the accelerations and there are many different designs and
manufacturing techniques, conceptually, all use a variation of the
spring mass system, shown in figure 1.
 In this system, when acceleration is applied, a small mass within the
accelerometer responds by applying a force to a spring, causing it to
stretch or compress. The displacement of the spring can be measured
and used to calculate the applied acceleration.
Types of accelerometers
 Accelerometers included for kinematic studies include
piezoelectric, piezeresistive, differential capacitor, all of
which implement the same basic principle of the spring
mass system.
 Piezoelectric accelerometers
 Piezoelectric accelerometers consist of a piezoelectric element
with a seismic mass.
 When the sensors undergo acceleration, the seismic mass
causes the piezoelectric element to bend.
 This change causes a displacement charge to build up on one
side of the sensor, which results in a variable output voltage
signal that is proportional to the applied acceleration.
Types of accelerometers
 Piezoresistive accelerometers
 These accelerometers are typically manufactured from a surface
micromachined polysilicon structure.
 On this sits polysilicon springs (arranged in a Wheatstone
configuration) whose electrical resistance changes as the
acceleration forces are applied.
 The acceleration is proportional to the resulting voltage.
 Differentiable capacitor accelerometers
 These operate on the principle of change of capacitance is
proportional to applied acceleration.
 They use a differentiable capacitor with central plates attached to
the moving mass and fixed external plates.
 The applied acceleration unbalances the capacitor.
 This results in the output wave for the accelerometer.
Acquiring the correct
accelerometer data
 The output of an accelerometer worn on the body is
dependent on four factors as listed as follows:
 Position at which it is placed,
 Its orientation at this location,
 The posture of the subject and
 The activity being performed by the subject.
 If the subject or patient is at rest the output of the
accelerometer, is determined by its inclination relative to the
gravitational vector.
 If the orientation of the accelerometer relative to the person is
known, then the resulting accelerometer recordings can be
used to determine the posture of the subject relative to the
vertical or gravitational direction.
Acquiring the correct
accelerometer data
 The main acceleration components are due to the
movement of the body.
 The body-generated linear, centripetal and coreolis
accelerations are the main factors that constitute both the
translational and rotational body movements.
 Other spurious resulting accelerations, such as artifact
due to soft tissue movement or external vibrations may
also be present in the accelerometer signal, but can be
minimized through careful instrument placement and
signal filtering.
Requirements for accelerometric
monitoring of human movements
 The accelerations generated during human movement
vary across the body and depend on the activity being
performed.
 Accelerations increase in magnitude from the head to
the ankle, and are generally greatest in the vertical
direction (Bhattacharya et al 1980), although the
accelerations in the other two directions cannot be
neglected (Lafortune 1991).
Medical assessment of human
movement
 Some analyses, such as gait analysis, under controlled
laboratory settings can be a useful tool in detecting
underlying causes of gait abnormalities; However,
 Analyses in a laboratory setting do not reveal functional
subject variability in everyday activity patterns.
 Accelerometers have become the preferred choice for
continuous, unobtrusive and reliable method in human
movement detection and monitoring.